Scanning electron microscope

ABSTRACT

The present invention is intended to prevent the deterioration of resolution due to increase in off-axis aberration resulting from the deviation of a primary electron bean from the optical axis of a scanning electron microscope. A scanning electron microscope is provided with an image shifting deflector system including two deflectors disposed respectively at upper and lower stages. The deflector disposed at the lower stage is a multipole electrostatic deflecting electrode and is disposed in an objective. Even if the distance of image shifting is great, an image of a high resolution can be formed and dimensions can be measured in a high accuracy. The SEM is able to achieving precision inspection at a high throughput when applied to inspection in semiconductor device fabricating processes that process a wafer having a large area and provided with very minute circuit elements.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a scanning electron microscopethat scans the surface of a specimen with an electron beam and forms atwo-dimensional electron image representing the shape or composition ofthe surface of the specimen through the detection of secondary signalsproduced by the specimen. More particularly, the present inventionrelates to a scanning electron beam microscope suitable for formingelectron beam images of a high resolution at a high throughput byrapidly moving an observation point to tens of test positions on asemiconductor wafer as a specimen.

[0002] A scanning electron microscope (hereinafter abbreviated to “SEM”)accelerates electrons emitted by an electron source of a heatingelectron emission type or a field electron emission type, collimates theaccelerated electrons in a fine electron beam, i.e., a primary electronbeam using an electrostatic lens or a magnetic field lens, scans aspecimen two-dimensionally with the primary electron beam, detectssecondary electrons generated by the specimen irradiated with theprimary electron beam or secondary signal electrons, i.e., reflectedelectrons, and forms a two-dimensional electron image by applyingintensities of detection signals as brightness modulating inputs to acathode-ray tube (abbreviated to “CRT”) that is scanned in synchronismwith a scanning operation using the primary electron beam.

[0003] Device miniaturization has progressively advanced in thesemiconductor industry in recent years, and optical microscopes forinspection in semiconductor device fabricating processes and testprocesses have been replaced by SEMs. The SEM uses an electron beam fordimension measurement and testing electrical operations. When observingan insulating specimen, such as a wafer that is used in thesemiconductor industry, is observed with a SEM, a low accelerationvoltage of 1 kV or below must be used not to charge the insulatingspecimen. Generally, the resolution of a general SEM using a lowacceleration voltage of 1 kV is about 10 nm. As the miniaturization ofsemiconductor devices advances, demand for SEMs capable of formingimages in a high resolution by using a low acceleration voltage hasincreased. A retarding system and a boosting system were developed andproposed in, for example, Japanese Patent Laid-open No. Hei 9-171791 tomeet such demand. Those previously proposed systems enable observationin a resolution of about 3 nm under optimum conditions for observation.

SUMMARY OF THE INVENTION

[0004] When a SEM is used for the inspection of a semiconductor deviceduring semiconductor device fabricating processes or a completedsemiconductor device, capability of rapidly moving an observation pointto tens of inspection positions on a semiconductor wafer is aprerequisite of the SEM for the improvement of the throughput of aninspection process. Therefore, a stage capable of rapid movement hasbeen used. However, the positioning accuracy of the stage is on theorder of several micrometers. Mechanical control of the position of thestage in an accuracy on the order of nanometers is economicallyinfeasible and is practically difficult in respect of moving speed.Therefore, to position the stage in a high accuracy higher than severalmicrometers, there is adopted an image shifting system that shiftselectrically the coordinates of the scanning center of a primaryelectron beam. In some cases, since the coordinates are shifted by adistance as long as several micrometers, the image shifting systememployed in the conventional SEM deteriorates resolution when thedistance of shift is great.

[0005] According to the present invention it is an object of the presentinvention to provide a SEM capable of image shifting an image withoutcausing significant deterioration of resolution.

[0006] With the foregoing object in view, the present invention providesa SEM comprising: an electron source, an image shifting deflector systemincluding two deflectors disposed respectively at upper and lower stagesto shift an irradiation position of a primary electron beam emitted bythe electron source on a specimen; and an objective that focuses theprimary electron beam; wherein the objective has a lens gap openingtoward the specimen, and the deflectors disposed at the lower stage onthe side of the specimen forms a deflecting electric field in a regioncorresponding to an effective principal plane of the objective.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is an overall, schematic view of a SEM in a preferredembodiment according to the present invention provided with an imageshifting deflector of a lower stage provided with an electrostaticdeflecting electrode;

[0008]FIG. 2 is a diagrammatic view of assistance in explainingdeflecting force and a conception of deflecting force cancellation atthe time of image shifting;

[0009]FIG. 3 is an overall, schematic view of a SEM provided with a Wienfilter that nullifies the off-axis chromatic aberration;

[0010]FIG. 4 is an overall, schematic view of a SEM additionallyprovided with an image shifting deflector according to the presentinvention;

[0011]FIG. 5 is a top plan view of an image shifting deflector of anupper-stage according to the present invention;

[0012]FIG. 6 is a top plan view of an octupole electrostatic deflectoremployed as an image shifting deflector at a lower stage in the presentinvention;

[0013]FIG. 7 is a schematic view of an image shifting deflector employedin a SEM in a second embodiment according to the present invention;

[0014]FIG. 8 is a schematic view of an image shifting deflector employedin a SEM in a third embodiment according to the present invention;

[0015]FIG. 9 is a view of assistance in explaining the irregularity ofbrightness when a specimen is observed by a SEM in a high magnification;

[0016]FIG. 10 is a schematic view of assistance in explaining a path ofsecondary electrons when an image is shifted in a SEM; and

[0017]FIG. 11 is a typical view of a deflection range of an imageshifting deflector.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018]FIG. 1 shows a SEM in a first embodiment according to the presentinvention. A cathode 4 emits electrons when a beam voltage 6 is appliedacross the cathode 4 and an emission control electrode 5. The electronsthus emitted are accelerated (decelerated in some cases) by the emissioncontrol electrode 5 and an anode 8 held at a ground voltage. Anacceleration voltage for accelerating a primary electron beam 1 is equalto an electron gun acceleration voltage 7. The primary electron beam 1accelerated by the anode 8 is gathered by a condenser lens 9. Angle ofdivergence of the primary electron beam 1 or beam current is determinedby a diaphragm 11 disposed below the condenser lens 9. A knob 12 isoperated for centering the diaphragm 11.

[0019] The primary electron beam 1 having passed the diaphragm 11 isdeflected by image shifting deflectors 20 and 30 having a scanningdeflection function for image shifting, and is moved on a specimen 13for two-dimensional scanning. The deflecting intensities of thedeflectors 20 and 30 are adjusted so that the primary electron beam 1travels straight through an objective 10. A deflection function forscanning and an image shift deflecting function are provided bysimultaneously applying a scanning deflection component and an imageshifting deflection component to the deflectors 20 and 30 by adeflection control power supply 40 to input the deflecting intensitiesof the deflectors 20 and 30.

[0020] A focusing magnetic field created by the objective acts on theprimary electron beam 1 so as to deflect the primary electron beam 1 indirections perpendicular to the direction of travel of the primaryelectron beam 1 to deflect the primary electron beam for image shiftingso that the primary electron beam deviates greatly from the optical axisof the objective 10. Thus, the deflection of the primary electron beam 1causes off-axis aberration. Such off-axis aberration can be suppressedby a deflector that deflects the primary electron beam 1 so as to canceldeflection caused by the objective 10. However, disposition of adeflector in the objective 10 is subjected to many physicalrestrictions. This SEM employs the objective 10 having a lens gapopening toward the specimen, i.e., an objective having a lower magneticpole having an aperture greater than that of an upper magnetic pole andcapable of creating a maximum focusing magnetic field in the vicinity ofthe specimen disposed below the objective. Thus, the effective principalplane of the objective is positioned on a level different from that ofthe objective or on a level that places only few physical restrictionsto facilitate the disposition of an electrostatic deflecting electrode.The SEM shown in FIG. 1 has an optical system of a short overall lengthbecause only the deflector 20 needs to be disposed between the condenserlens 9 and the objective 10.

[0021] The primary electron beam 1 is decelerated by a deceleratingelectric field created between the objective 10 and the specimen 13 byapplying a negative retarding voltage 15 through a stage 14 to thespecimen 13 and is collimated by the collimating action of the objective10.

[0022] In this embodiment, the upper deflector 20 is a magneticdeflector and the lower deflector 30 is an electrostatic deflector. Theupper deflector 20 may be an electrostatic deflector. Similarly, thelower deflector 30 may be a magnetic deflector. However, since only anarrow space is available in the vicinity of the objective 10, it isproper to use an electrostatic deflector as the lower deflector 30.

[0023] Image shifting deflecting intensity I_(IS) given to the upperdeflector 20 by the deflection control power supply 40 is expressed bythe following expression.$I_{IS} = {K_{1}\frac{\sqrt{V_{acc}}}{{LM}_{sem}}}$

[0024] where K₁ is conversion coefficient for converting deflectionsensitivity, M_(SEM) is the magnification of the SEM, V_(acc) isacceleration voltage for accelerating the primary electron beam 1, L isthe distance between the upper deflector 20 and the specimen 13. Imageshifting deflecting intensity V_(IS) given to the lower deflector 30 isexpressed by the following expression.

V_(IS)=K₂I_(IS)

[0025] where K₂ is conversion coefficient for converting deflectionsensitivity. The angle between the direction of a magnetic field createdby the upper deflector 20, i.e., a magnetic deflector, and that of anelectric field created by the lower deflector 30, i.e., an electrostaticdeflector, is about 90°. This angle between the directions differs from90° when a magnetic field is created above the objective 10. This anglecan be previously determined by numerical simulation or experiments.

[0026] Secondary signal electrons 2 are generated when the specimen 13is irradiated with the primary electron beam 1. The secondary signalelectrons 2 include secondary electrons and reflected electrons. Theelectric field created in a space between the objective 10 and thespecimen 13 acts as an acceleration electric field on the secondarysignal electrons 2. Therefore, the secondary signal electrons 2 areattracted to the electron beam passing aperture of the objective 10. Thesecondary signal electrons 2 travel upward being subjected to thefocusing action of the magnetic field of the objective 10. The secondarysignal electrons having high energy collide against a conversionelectrode 16, whereby secondary electrons 3 are emitted. A positive highvoltage of about 10 kV is applied to a scintillator 17. The scintillator17 attracts (deflects) the secondary electrons 3 and emits light. Asecondary electron detector, not shown, that detects secondary electronsguides the light emitted by the scintillator 17 by a light guide to aphotomultiplier, the photomultiplier converts the light into acorresponding electric signal, the electric signal is amplified and theamplified electric signal is used for the brightness modulation of aCRT.

[0027] The principle and advantages of the SEM in this embodiment willbe specifically described with reference to FIGS. 2, 9 and 10. FIG. 9shows a general SEM for semiconductor wafer inspection, and paths ofsecondary electrons. FIG. 9 shows an observation mode in which thespecimen 13 is observed at a high magnification and image shifting isnot used. In this observation mode, the primary electron beam 1 falls onthe specimen 13 at a position very close to the optical axis and hencethe high-energy accelerated secondary electrons 2 fall in regions nearthe optical axis on the conversion electrode 16. Since the conversionelectrode 16 is provided with a central aperture through which theprimary electron beam 1 passes, some secondary electrons 2 a travelthrough the central aperture of the conversion electrode 16 and are notdetected. Consequently, an image having irregular brightness is formed.FIG. 10 shows an observation mode in which the specimen 13 is observedat a high magnification and image shifting is used. Secondary electrons2 emitted from a position to which an image is shifted pass through aretarding electric field, not shown, and the objective 10, and travelalong a path slightly deviating from the optical axis and fall in aregion not including the central aperture on the conversion electrode16.

[0028] Since the range of deflection of the primary electron beam 1 islimited by the central aperture of the conversion electrode 16, thecentral aperture of the conversion electrode 16 cannot be excessivelyreduced. The diameter of the central aperture of the conversionelectrode 16 is, for example, 3 mm. Since the optical magnification ofthe objective 10 is, for example, 50×, an image shifting distance, forexample, on the order of 60 μm is necessary to enable the secondaryelectrons fall in regions not including the central aperture on theconversion electrode 16. On the other hand, ordinary image shiftingdeteriorates resolution by off-axis aberration when the image shiftingdistance is greater than 10 μm. Therefore, it is difficult to observe animage of a high resolution when image shifting is executed.

[0029] The SEM in this embodiment employs a multipole electrostaticdeflector as the lower image shifting deflector and forms theelectrostatic deflector on the effective principal plane of theobjective to achieve the efficient detection of the secondary electronswithout causing significant deterioration of resolution, even if animage shifting amount is great. In the SEM shown in FIG. 1, a magneticlens is formed by the objective 10 and an electrostatic lens is formedby the retarding voltage 15 applied to the specimen 13 in the vicinityof the specimen 13. Although the magnetic and the electrostatic lens areshown separately in FIG. 2, actually, the magnetic and the electrostaticlens are superposed. FIG. 2A shows deflecting forces exerted by themagnetic and the electrostatic lens on the primary electrons travelingalong an off-axis path. The deflecting force FB0 of the objective actsin a rotating direction and the deflecting force FE0 of theelectrostatic lens acts in a radial direction. The deflecting force FB0is always greater than the deflecting force FE0 (FB0>FE0). FIG. 2B showsthe so-called moving objective that cancels deflecting forces bysuperposing lateral deflecting electric field FE1 and a magnetic fieldFB1 on the lens electric field and magnetic field. Since the deflectingforces are cancelled individually, i.e., FB0+FB1=0 and FE0+FE1 =0,off-axis aberration is suppressed to the least extent. FIG. 2C shows thecancellation of deflecting force only by the deflecting electric field.Deflecting force acting on the primary electrons can be cancelled by:FB0+FE2=0 and FE0+FB1=0. Since the secondary electrons travel in thereverse direction, the deflecting force of the magnetic field isreversed. Generally, FB0+FE2=2×FE0 and FE0+FE1=0, and a comparativelylarge deflecting force remains. FIG. 2D shows the cancellation of thedeflecting force only by superposition of the deflecting magnetic field.For the primary electrons, FB0 +FB1=0 and FE0+FE1=0. For the secondaryelectrons, the deflecting force of the magnetic field is reversedbecause the secondary electrons travel in the reverse direction.Generally, FB0+FE2=0 and FE0+FB2=2×FE0, and a comparatively smalldeflecting force remains. As obvious from FIGS. 2A to 2D, thesuperposition of the deflecting electric field shown in FIG. 2C isadvantageous to meet both the elimination of the off-axis aberration ofthe primary electrons and the deflection of the secondary electrons.

[0030] When observing an image with the image shifted by a fixeddistance on the basis of the foregoing principle by the SEM in thisembodiment, the secondary electrons 2 are caused to travel along a pathextending apart from the optical path so that most of the secondaryelectrons fall in a region not including the central aperture of theconversion electrode 16 on the conversion electrode 16, to suppressoff-axis aberration due to image shifting and to improve secondaryelectron detecting efficiency. In some cases, image shifting deflectionimproves the secondary electron detecting efficiency in a SEM employingthe retarding technique and it is desirable to set an observation pointwith awareness of such a fact.

[0031] It is possible to prevent the secondary electrons from passingthe central aperture of the conversion electrode 16 by disposing anenergy filter 60 including a plurality of layers of meshes below theconversion electrode 16 with respect to the traveling direction of theprimary electron beam, whereby energy discriminating ability isimproved. In the SEM in this embodiment, a secondary electron detector,not shown, may be interposed between the energy filter 60 and theobjective 10 to catch all the secondary electrons that collide againstthe meshes of the energy filter 60 and do not reach the conversionelectrode 16.

[0032] When there is not any retarding electric field or the retardingelectric field is sufficiently small, only the reflected electrons passthe electron beam passing aperture of the objective 10. The reflectedelectrons have high energy. Positions at which the reflected electronsfall on the conversion electrode 16 are dependent on angle at which theelectrons are reflected by the specimen 13 and energy of the reflectedelectrons. Therefore, information represented by the selected reflectedelectrons can be obtained in a high sensitivity by disposing an aperturefilter 62 below the conversion electrode 16 with respect to thetraveling direction of the primary electron beam. When the reflectedelectrons reflected in a substantially perpendicular direction areselected, an image of high contrast of a specimen having a specificatomic number can be observed in a high resolution. In the conventionalSEM, the path of the reflected electrons and the path of the primaryelectrons overlap each other and hence the detection of the reflectedelectrons is difficult.

[0033] Substantially the same effect can be expected by making only apart of the conversion electrode 16 emit secondary electrons instead ofemploying the aperture filter 62. In such a case, it is preferable tocoat the conversion electrode 16 excluding a part of the same withcarbon that emit secondary electrons at a low efficiency.

[0034]FIG. 3 shows a SEM in a second embodiment according to the presentinvention. In the SEM in the first embodiment, the objective 10 causeslight off-axis aberration because deflecting force is exerted on thesecondary electrons 2. The off-axis aberration is a significant problemthat affects adversely to observation in a high resolution. In the thirdembodiment, a Wien filter 62 adjusted so as to cancel off-axisaberration caused by an objective 10 is disposed on the side of anelectron source with respect to a conversion electrode 16 to avoid theproblem attributable to off-axis aberration.

[0035]FIG. 4 shows a SEM in a third embodiment according to the presentinvention. The SEM in the third embodiment is provided with, in additionto two scanning deflectors 18 and 19 of a general SEM disposed at twostages, image shifting deflectors 20 and 30 in accordance with thepresent invention. A primary electron beam 1 traveled through adiaphragm 11 is deflected for two-dimensional scanning on a specimen 13by the scanning deflectors 18 and 19. A deflection control power supply40 gives a deflecting intensity I_(IS) corresponding to a shiftingdistance to the upper image shifting deflector 20 for image shifting andgives a deflecting intensity V_(IS) adjusted so as to make a primaryelectron beam 1 travel straight through an objective 10 to the lowerimage shifting deflector 30.

[0036] Thus, the image shifting deflectors 20 and 30 can be easilyincorporated into the general SEM to improve image shifting function,resolution and accuracy of dimensional measurement.

[0037] The image shifting deflectors 20 and 30 will be described withreference to FIGS. 4 and 5. The upper image shifting deflector 20 is thesame in construction as a conventional scanning deflector. The upperimage shifting deflector 20 has scanning coils 21 to 24 are cosinedistributed winding coils to create a uniform deflecting magnetic fieldaround the optical axis of the SEM. The four quadrant coils are disposedin a circle. Coil currents are regulated in proportion to the cosine ofthe angle φ between an electron beam deflecting direction and theposition of the scanning coils to deflect the primary electron beam 1 ina desired direction. Usually, currents of the same absolute value andopposite directions are supplied to the opposite scanning coils 21 and23, respectively. Therefore, currents can be supplied to both thescanning coils 21 and 23 from a single power supply by connecting thescanning coils 21 and 23 to the power supply in opposite ways ofconnection, respectively. Similarly, currents can be supplied to theopposite scanning coils 22 and 24 by a single power supply.

[0038] The lower image shifting deflector 30 is an octupoleelectrostatic deflector. Since the lower image shifting deflector 30 isdisposed in a narrow space between the objective 10 and the specimen 13,the lower image shifting deflector 30 is formed in the shape of a disk.Although the octupole electrostatic deflector can be constructed byassembling eight ⅛ electrode sectors, the octupole electrostaticdeflector is formed by the following method to assemble the same in ahigh accuracy and to reduce assembling costs. An electron beam passingaperture is formed in an insulating disk of several millimeters inthickness. Insulating slits are formed in the insulating disk so as toextend radially from the electron beam passing aperture. Eightelectrostatic deflecting electrodes 31 to 38 are formed by coating thefront and the back surface of a part of the disk around the electronbeam passing aperture and the side surfaces of the electron beam passingaperture and the insulating slits with a conductive material by a vapordeposition process or a plating process. Voltages to be applied to theelectrodes 31 to 38 are regulated in proportion to the cosine of theangle θ between an electron beam deflecting direction and the positionof the electrodes 31 to 38 to deflect an electron beam by a desireddistance in a desired direction. An angular displacement Δφ correspondsto the angle of rotation of a primary electron beam caused by a lensmagnetic field created in a space between the upper image shiftingdeflector 20 and the lower image shifting deflector 30.

[0039]FIGS. 7 and 8 show lower image shifting deflectors 30 suitable foruse in combination with an objective 10 having a principal plane on alevel above the bottom surface of the objective 10, i.e., a level in theelectron beam passing aperture of the objective 10. The lower imageshifting deflector 30 shown in FIG. 7 has a funnel-shaped insulatingbase plate and the base plate is inserted from above the objective 10 inthe electron beam passing aperture. A head part of the insulating baseplate is divided into eight divisions and coated with a conductivematerial by the foregoing method. A shielding electrode 39 prevents thecharging effect of an insulating part of the lower image shiftingdeflector 30 and the creation of a deflecting electric field in a regionnot affected by an objective magnetic field. The lower image shiftingdeflector 30 shown in FIG. 8 is inserted from below an objective 10 inan electron beam passing aperture formed in the objective 10. Theinsulating base plate of the lower image shifting deflector 30 has aflat, annular peripheral part and a cylindrical central part extendingfrom a central portion of the peripheral part. The peripheral part andthe central part of the base plate are divided into eight divisions, andcoated with a conductive material by the foregoing method. In somecases, the cylindrical part is extended not only toward the objective 10but also toward the specimen 13 according to the distribution of theobjective magnetic field. A deflection control power supply 40 applies avoltage to the lower image shifting deflector 30 relative to a groundpotential to deflect an electron beam. The surface electric field of thespecimen 13 can be adjusted by off-setting a reference potential by apower supply 49, which is effective in charging and adjusting surfacepotential for the observation of an insulating specimen. The lower imageshifting deflector 30 can be easily installed also when the SEM isprovided with a height measuring device that measures the height of thespecimen 13 by using a laser beam. A laser light source 51 projects alaser beam 52 obliquely to the specimen 13. The laser beam 52 reflectedby the specimen 13 is detected by a position sensor 53. The position ofthe reflected laser beam 52 on the position sensor 53 varies accordingto the height of the specimen 13. The variation of the height of thespecimen 13 is determined through the measurement of the variation ofthe position of the reflected laser beam 52 on the position sensor 53.The lower image shifting deflector 30, i.e., the octupole electrostaticdeflector, can be easily disposed so that the laser beam 52 and thereflected laser beam 52 may pass the insulating slits of the lower imageshifting deflector 30.

[0040] In the SEM in this embodiment, the upper image shifting deflectordeflects the electron beam off the optical axis taking the Lorentz forceof the objective into consideration, and the lower image shiftingdeflector executes electrostatic deflection of the electron beam so thatthe axial deviation of the electron beam by the Lorentz force may besuppressed and the electron beam travels straight toward the specimen.Therefore, off-axis aberration due to a large angle of deflection of theelectron beam can be suppressed and resolution can be improved.

[0041] Since an electrostatic deflector is used as the lower imageshifting deflector disposed between the lower magnetic pole of theobjective of an open lower magnetic pole type and the specimen, theelectron beam can be deflected without increasing the short focal lengthof the objective.

[0042] The SEM in this embodiment reduces aberration by employing theobjective having a short focal length and reduces off-axial aberrationby controlling the angle of deflection for image shifting.

[0043]FIG. 11 is a view of assistance in explaining a SEM in a fourthembodiment according to the present invention. FIG. 11 shows typically adeflection range 101 for image shifting. The SEM as shown in FIG. 1 isprovided with a controller, not shown. The controller sets values ofparameters including observation positions and magnification, andcontrols a mirror included in the SEM on the basis of the set values. ASEM for inspecting semiconductor wafers needs to observe a plurality ofpoints on the surface of a semiconductor wafer. Recipe specifyingconditions for the observation of the plurality of points are setbeforehand or the recipe is set manually.

[0044] A plurality of high-magnification observation regions 103 can beset in the deflection range 101 in which the electron beam is deflectedby image shifting deflectors 20 and 30. As mentioned above, mostsecondary electrons from a central point 102 of the deflection range101, i.e., a point corresponding to a primary electron beam 1, passthrough the aperture of a conversion electrode 16, and an image of aspecimen, having irregular brightness is formed. It is desirable toprovide the SEM with a sequence that inhibits setting of ahigh-magnification observation region 103 at the center 102 of thedeflection range 101. For example, when the SEM is provided with asequence that sets a desired high-magnification observation region amonglow-magnification images, the foregoing problem can be prevented bymaking the setting of a high-magnification observation region 103 at thecenter 102 of the deflection range impossible or by generating a warningrequesting moving the stage and resetting the high-magnificationobservation region 103. When the SEM is provided with recipe specifyingoperations for multiple-point observation, it is desirable to specifyconditions for controlling the stage so that the high-magnificationobservation region 102 may not be located at the center of thedeflection range, to generate a warning when conditions are set so as tolocate the high-magnification observation region 102 at the center ofthe deflection region or to inhibit the setting of such conditions. Theoperator is able to carry out operations for setting thehigh-magnification region at a position other than the center of thedeflection range 101 without depending on warnings or the like when atypical image of the deflection region 101 of the image shiftingdeflectors as shown in FIG. 11 is displayed on the screen of a display,not shown.

[0045] The SEM in this embodiment is able to form an image of a highresolution and to measure dimensions in a high accuracy even if thedistance of image shifting is great. In particular, in semiconductordevice fabricating processes that process a wafer having a large areaand provided with very minute circuit elements, the SEM is able toachieve precision inspection at a high throughput.

1-17 Cancelled
 18. A scanning electron microscope comprising: anelectron source; an objective lens focusing a primary electron beamemitted from the electron source on a specimen; and a deflectordeflecting the primary electron beam to an off-axis of the objectivelens; wherein the objective lens has a lens gap opening toward thespecimen, and a plurality of electrostatic deflecting electrodes whichare disposed between the objective lens and the specimen deflect theprimary electron beam as reducing aberration generated by an irradiationof the primary electron beam off-axis of the objective lens.
 19. Thescanning electron microscope according to claim 18, wherein theelectrostatic deflecting electrodes deflect the primary electron beam toa direction opposite a deflecting direction of the objective lens. 20.The scanning electron microscope according to claim 18, wherein theelectrostatic deflecting electrodes create an electric field thatsuppresses off-axis deviation of the primary electron beam caused by amagnetic field created by the objective lens.
 21. The scanning electronmicroscope according to claim 18, the deflector serves also as scanningdeflector for deflecting the primary electron beam to scan the specimenwith the primary electron beam.
 22. The scanning electron microscopeaccording to claim 18, wherein the electrostatic deflecting electrodesare an octupole deflector.
 23. The scanning electron microscopeaccording to claim 18, wherein: the electrostatic deflecting electrodeshave an insulating base plate provided with a primary electron beampassing aperture and insulating slits extending radially from theelectron beam passing aperture, and at least a portion of both oppositesurfaces of insulating plate around the electron beam passing aperture,and a side surface of the electron beam passing aperture, and theinsulating slits are coated with conductive films.
 24. The scanningelectron microscope according to claim 23, wherein the insulating baseplate has a conductive, cylindrical part formed around the primaryelectron beam passing aperture, and the conductive, cylindrical part ofthe insulating base plate is inserted in a primary electron beam passingaperture of the objective lens.
 25. The scanning electron microscopeaccording to claim 18 further comprising a secondary signal detector fordetecting a secondary signal produced by the specimen, said secondarysignal detector including a secondary electron conversion electrode toconvert highly accelerated electrons that are produced when the specimenis irradiated with the primary electron beam, into secondary electrons.26. The scanning electron microscope according to claim 18 furthercomprising: a conversion electrode that emits secondary electrons uponbombardment with electrons emitted by the specimen in response to thespecimen being irradiated with the primary electron beam; and asecondary electron detector that deflects the secondary electronsemitted by the conversion electrode off the axis of the primary electronbeam and detects the secondary electrons.
 27. The scanning electronmicroscope according to claim 26, wherein the conversion electrode emitsthe secondary electrons when a specific portion of conversion electrodeis bombarded by the electrons.
 28. The scanning electron microscopeaccording to claim 18 further comprising a Wien filter for controllingoff-axis aberration of the objective.
 29. A scanning electron microscopecomprising: an electron source; an objective lens focusing a primaryelectron beam emitted from the electron source on a specimen; and adeflector for deflecting the primary electron beam to an off-axis of theobjective lens; comprising: means for setting a deflecting point on theobjective lens; and means for warning or making impossible to set thedeflection point to the off-axis of the objective lens, when thedeflection point is set a point that a secondary electron detectingefficiency decreases in the off-axis deflecting area of the deflector.30. A scanning electron microscope comprising: an electron source; anobjective lens focusing a primary electron beam emitted from theelectron source on a specimen; and an image shifting deflectordeflecting an irradiation position of the primary electron beam on thespecimen; comprising: a display for displaying an area that a secondaryelectron detecting efficiency decreases in a deflecting area of theimage shifting deflector.